Process for converting a chlorinated alkane into a less chlorinated alkene

ABSTRACT

Process for converting a chlorinated alkane into at least one less chlorinated alkene by reaction with hydrogen in the presence of a catalyst comprising palladium and a metal selected from the group consisting of silver, gallium, indium, thallium, germanium, tin, lead, arsenic, antimony, bismuth and mixtures thereof.

This application is a 371 of PCT/EP95/04516, filed Nov. 15, 1995.

The invention relates to a process for converting a chlorinated alkaneinto at least one less chlorinated alkene by reacting the chlorinatedalkane with hydrogen in the presence of a catalyst comprising a metalfrom group VIII and another metal on a support.

TECHNOLOGY REVIEW

International applications WO-94/07828, WO-94/07827, WO-94/07823,WO-94/07821, WO-94/07820, WO-94/07819, and WO-94/07818 describeprocesses for converting various chlorinated alkanes into lesschlorinated alkenes by means of hydrogen in the presence of a bimetalliccatalyst comprising a metal from group VIII and a metal from group IBwhich are deposited on a support. European Patent Application EP-A-0 640574 describes the conversion of chlorinated alkanes into lesschlorinated alkenes in the presence of a bimetallic catalyst comprisingplatinum and a second metal, such as lanthanum, titanium, vanadium,chromium, manganese, iron, cobalt, nickel, copper, zinc, indium, tin orbismuth, on a support. In these known processes, the best degrees ofconversion of the chlorinated alkanes and the best selectivities asregards the production of alkenes are obtained with the bimetalliccatalyst platinum-copper on active charcoal. International ApplicationWO-94/07819 and European Patent Application EP-A-0 640 574 describe morespecifically processes for converting 1,2-dichloropropane intopropylene. It is evident therefrom that the bimetallic catalystsspecified above do not make it possible to obtain both a high degree ofconversion of 1,2-dichloropropane and a high selectivity for propylene.Moreover, these catalysts are initially of very low selectivity withrespect to the formation of propylene, with a large quantity of propanebeing produced. Because of this, these known catalysts are unsuitablefor generating propylene which can be used directly in a unit for theproduction of allyl chloride by chlorination of propylene. In effect,when a mixture comprising propylene and propane is recycled to the allylchloride production stage, 1-chloropropane and/or 2-chloropropane areformed by chlorination of propane, and these products are difficult toseparate from the allyl chloride. Another disadvantage of these knowncatalysts is their rapid deactivation. Consequently, the pretreatment ofthese catalysts using hydrogen chloride is necessary in order to improvetheir initial selectivity and their stability. The patent U.S. Pat. No.3,892,818 discloses a process for de-chlorinating 1,2-dichloropropaneusing hydrogen in the presence of a bimetallic rhodium-gold catalystsupported on alumina. This catalyst has a good activity and a long life,but the reaction product is essentially propane.

SUMMARY OF THE INVENTION

A process has now been found which does not have the above-describeddisadvantages and which makes it possible to convert chlorinated alkanesinto less chlorinated alkenes with a good selectivity and preferablywith a high degree of conversion without the catalyst either becomingrapidly deactivated over time or requiring pretreatment with hydrogenchloride.

The invention consequently relates to a process for converting achlorinated alkane into at least one less chlorinated alkene by reactingthe chlorinated alkane with hydrogen in the presence of a catalystcomprising a metal from group VIII and a metal M, on a support, which ischaracterized in that the metal from group VIII is palladium and themetal M is selected from the group consisting of silver, gallium,indium, thallium, germanium, tin, lead, arsenic, antimony, bismuth andmixtures thereof.

DETAILED DESCRIPTION OF THE INVENTION

The chlorinated alkane employed in the process according to theinvention is an alkane comprising at least one chlorine atom. Goodresults have been obtained with acyclic chlorinated alkanes and, moreespecially, with acyclic chlorinated alkanes of general formula C_(n)H_(2n+2-x) Cl_(x) in which n is an integer from 2 to 6 and x is aninteger from 1 to (2n+2). Chloropropanes are particularly advantageous,and dichloropropanes and trichloropropanes are more especially so.1,2-dichloropropane is very particularly advantageous.

The term less chlorinated alkene is intended to denote an alkene inwhich the number of carbon atoms corresponds to the number of carbonatoms of the chlorinated alkane employed and which has at least onechlorine atom less than the chlorinated alkane employed. The lesschlorinated alkene as defined in the present invention may thereforecontain no chlorine atoms. In the case of a chlorinated alkane ofgeneral formula C_(n) H_(2n+2-x) Cl_(x) in which x=1 to (2n+2), thealkene formed in the process according to the invention thereforecorresponds to the general formula C_(n) H_(2n-y) Cl_(y) in which yvaries from 0 to 2n, without being higher than (x-1). In the processaccording to the invention, the reaction of the chlorinated alkane withhydrogen may produce a single less chlorinated alkene as defined aboveor a mixture of two or more less chlorinated alkenes.

The catalyst employed in the process according to the inventioncomprises palladium and at least one metal M selected from the groupconsisting of silver, gallium, indium, thallium, germanium, tin, lead,arsenic, antimony and bismuth, on a support. The metal M is preferablyselected from silver, tin, lead, thallium and bismuth. Good results havebeen obtained when the metal M is tin. Excellent results have beenobtained when the metal M is silver. The catalyst preferably consistsessentially of palladium and a metal M on the support. The palladium andthe metal M may be in the elemental state or in the form of a compound,such as a salt or an oxide. The catalyst preferably comprises palladiumand the metal M in the elemental state.

As catalyst support, use is commonly made of a porous support such asthose which are currently used with the catalysts employed inhydrogenation reactions. Examples of such supports are active charcoal,alumina, silica, titanium oxide, magnesium oxide, zirconium oxide,lithium aluminate and silica-alumina. The preferred support is activecharcoal.

The quantity of palladium on the support is advantageously at least0.05%, preferably at least 0.15%, by weight relative to the weight ofthe support. Commonly, the quantity of palladium does not exceed 10% byweight relative to the weight of the support. Preferably it does notexceed 5%.

The quantity of metal M on the support is advantageously at least 0.05%,preferably at least 0.15%, by weight relative to the weight of thesupport. Commonly, the quantity of this metal M does not exceed 10% byweight relative to the weight of the support. Preferably it does notexceed 5%.

The ratio by weight of the palladium to the metal M is preferably atleast 0.05. It is particularly preferred for this ratio by weight to beat least 0.1. It is more particularly preferred for this ratio by weightto be at least 0.25. Preferably, the ratio by weight of the palladium tothe metal M does not exceed 20. It is particularly preferred for thisratio not to exceed 10. It is more particularly preferred for this rationot to exceed 4.

In a specific embodiment of the process according to the invention, inwhich the metal M is silver, the weight ratio of palladium to silver isvery particularly preferably at least 0.4. In this embodiment of theinvention, the weight ratio of palladium to silver preferably does notexceed 2.5.

The catalyst may additionally, if appropriate, comprise at least oneadditional metal which is selected from metals from group IB, IIB, IIIA,IVA, VA and VIII, in the elemental stage or in the form of a compound ofthis metal (the groups are designated in accordance with the CASnomenclature as reproduced in the CRC Handbook of Chemistry and Physics,75th edition, 1994-1995, D. R. Lide, cover page). If appropriate, thequantity of this additional metal does not exceed 50% by weight of theoverall weight of palladium and the metal M.

The metals of the catalyst employed in the process according to theinvention can be deposited on the support by impregnating the latterwith one or more solutions containing the metal constituents of thecatalyst. The impregnating solutions are preferably aqueous saltsolutions. The salts used for this purpose are, in particular,chlorides, nitrates, acetates or ammonia complexes. According to apreferred variant of the process according to the invention, a catalystis employed which is obtained by two successive impregnations. In thiscase, the support is first of all impregnated with a solution comprisingpalladium, dried, then impregnated with a solution comprising the metalM, and dried again. Commonly, the impregnated and dried support issubjected to heat treatment in a reducing atmosphere such as, forexample, hydrogen at a temperature of at least 100° C. and preferablyless than or equal to 400° C. The heat treatment of the impregnatedsupport can be carried out prior to the use of the catalyst in theprocess or at the same time as the chlorinated alkane and hydrogen areemployed in the process.

In the process according to the invention, the molar ratio of hydrogento chlorinated alkane is preferably at least 0.1, more particularly atleast 0.5. This ratio preferably does not exceed 40. With particularpreference, it does not exceed 20.

In the process according to the invention, the hydrogen reacts with thechlorinated alkane to produce at least one less chlorinated alkene, asset out above. The hydrogen can if appropriate be mixed with anothergas, which is inert under the reaction conditions of conversion of thechlorinated alkane into less chlorinated alkene. The other gas used maybe a gas from the group of the inert gases proper, such as helium, or agas which does not intervene in the abovementioned reaction, such ashydrochloric acid or an alkene. In the case where the inert gas selectedis an alkene, it is preferably the alkene or one of the alkenes formedby the reaction of the chlorinated alkane with hydrogen. The fraction byvolume of hydrogen is preferably at least 5% of the total volume ofhydrogen and the other gas. With particular preference, the hydrogenfraction is at least 10% of the total volume.

The process according to the invention can be carried out in liquidphase or in gaseous phase. The process according to the invention ispreferably performed in the gaseous phase. The process takes placepreferably at a temperature of at least 150° C., more particularly atleast 200° C. The temperature generally does not exceed 450° C.Preferably it does not exceed 400° C. The pressure at which the processis carried out is not critical per se. Generally, a pressure of at least1 bar is employed. In general, the pressure does not exceed 30 bar.Preferably it does not exceed 10 bar.

In the case where the process according to the invention is carried outin the gaseous phase, the mean contact time between the gases employedand the catalyst, i.e. the ratio between the volume occupied by thecatalyst and the total feed flow rate, measured at the temperature andpressure of the reaction, is preferably at least 0.5 second, moreparticularly at least 1 second. The contact time preferably does notexceed 30 seconds. With particular preference, the contact time does notexceed 20 seconds.

The process according to the invention makes it possible to obtain ahigh degree of conversion of the chlorinated alkane and a very largeselectivity for less chlorinated alkene. The process according to theinvention additionally makes it possible to obtain a good selectivityfor the less chlorinated alkenes, without any notable formation ofalkanes or chlorinated alkanes, and it achieves these advantages rightfrom the moment when the catalyst is first employed and withoutpretreatment of the catalyst with hydrogen chloride. The processaccording to the invention, moreover, has the advantage thatdeactivation of the catalyst over time is particularly slow comparedwith the deactivation of known catalysts from the prior art.

In the specific case where the chlorinated alkane employed in theprocess according to the invention is 1,2-dichloropropane, propylene isobtained with a good selectivity and a good degree of conversion. Theinvention therefore relates in particular to a process for obtainingpropylene by reacting 1,2-dichloropropane with hydrogen in the presenceof a catalyst comprising palladium and a metal M selected from the groupconsisting of silver, gallium, indium, thallium, germanium, tin, lead,arsenic, antimony, bismuth and mixtures thereof on a support.

The process according to the invention finds very advantageousapplication in the conversion of chloropropanes, and more particularlyof chloropropanes which are formed as by-products in the production ofallyl chloride by chlorination of propylene and/or in the production ofepichlorohydrin by hypochlorination of allyl chloride. Particularexamples of chloropropanes which are by-products in these productionprocesses are 1,2-dichloropropane and 1,2,3-trichloropropane. Thechloropropanes employed in this application of the process according tothe invention may contain a small quantity, generally less than 5% byweight, of other products, especially products which occur in theproduction of allyl chloride and/or of epichlorohydrin, and moreparticularly chloropropenes, such as 1,3-dichloropropene,2-chloropropene and allyl chloride. This particular application of theprocess according to the invention is particularly advantageous since itmakes it possible to obtain propylene which contains only a very smallquantity of propane, generally less than 3% and most frequently lessthan 1%, which can thus be recycled directly to the stage of productionof allyl chloride by chlorination of propylene. The use in the allylchloride production stage of a propylene of low propane content makes itpossible to limit the quantity of 1-chloropropane and/or 2-chloropropanewhich are formed by chlorination of propane and which are difficult toseparate from allyl chloride.

EXAMPLES

The invention is illustrated more fully by the following examples.

Example 1 (in accordance with the invention)

In this example, a Pd-Ag catalyst was employed on a support of activecharcoal.

a) Preparation of the catalyst on the support

10 g of active charcoal (grade NC 35 sold by the company CECA) having apore volume of 0.5 ml/g were introduced into a round-bottomed flask with3.5 ml of water and 1.5 ml of a solution containing 0.10 g Pd/ml(solution of PdCl₂ in 6N HCl). After a period of 15 minutes at roomtemperature, the impregnated active charcoal was dried under vacuum at80° C. After cooling to room temperature, 6 ml of a solution containing0.05 g of Ag/ml (AgCl in an aqueous ammonia solution containing 25% byweight of NH₃) were introduced into the flask. After a period of 15minutes at room temperature, the impregnated active charcoal was driedinitially under vacuum at 80° C. and then under a helium atmosphere for1 h at 120° C. and for 1 h at 280° C. The impregnated and dried activecharcoal was then treated at 280° C. for 4 h with hydrogen. The catalystthus obtained comprised 1.5% by weight of Pd and 3% of Ag relative tothe weight of active charcoal employed. X-ray diffraction analysis ofthe catalyst showed that the metals were in part present in the form ofalloys comprising between 20 and 50 atom % of silver, the particle sizebeing from 3 to 10 nm.

b) Conversion of 1,2-dichloropropane

3.43 g (7.50 cm³) of the catalyst described above were introduced into areaction tube (internal diameter=0.8 cm). The reactor containing thecatalyst was then fed continuously at a rate of 2.6 l/h (s.t.p.) with1,2-dichloropropane and with 10.3 l/h (s.t.p.) of hydrogen at 345° C.under 1.5 bar for several hours. The residence time was assessed asbeing 1.39 s.

At various intervals of time, a sample of the products continuouslyleaving the reactor was taken and was analysed by gas chromatography,and the degree of conversion of 1,2-dichloropropane and the selectivityfor propylene (defined as the molar fraction of 1,2-dichloropropanereacted which has been converted to propylene) were measured. Theresults of the measurements are reproduced in Table I. This table showsthat, after more than 150 h of operation, the catalyst is still asactive and selective as at the beginning. After approximately 8 days ofoperation, the catalyst still makes it possible to obtain a degree ofconversion of greater than approximately 95%. At this time,approximately 711 kg of 1,2-dichloropropane have been converted per kgof catalyst.

Example 2 (not in accordance with the invention)

1,2-dichloropropane was converted under the same operating conditions asthose described in Example 1, but using 3.67 g (7.50 cm³) of a catalystcomprising 2.7% by weight of Pt and 1.8% by weight of Cu relative to theweight of active charcoal. This catalyst was applied to the same supportand under the same operating conditions as in Example 1, using aqueoussolutions of H₂ PtCl₆.6H₂ O and CuCl₂.2H₂ O.

After various intervals of time, the degree of conversion of1,2-dichloropropane and the selectivity for propylene were measured asin Example 1. The results of these measurements are likewise reproducedin Table I.

                  TABLE I                                                         ______________________________________                                               EXAMPLE 1     EXAMPLE 2                                                       (1.5% Pd-3.0% Ag)                                                                           (2.7% Pt-1.8% Cu)                                        DURATION conversion                                                                              selectivity                                                                             conversion                                                                            selectivity                              (h)      %         mol %     %       mol %                                    ______________________________________                                        0.50     99.80     87.12     99.96   80.69                                    3.50     99.93     90.42     99.94   84.49                                    5        99.95     91.05                                                      5.50                         99.93   85.47                                    10.50                        99.92   86.90                                    12       99.97     92.18                                                      18.50                        99.89   88.13                                    20       100       92.86                                                      36       100       93.68                                                      38.50                        99.76   89.82                                    52       100       93.70                                                      54.50                        99.54   90.84                                    75       100       93.97                                                      77.50                        98.53   90.69                                    99       99.95     94.89                                                      101.50                       95.35   92.99                                    123      99.95     94.83                                                      125.50                       85.54   93.51                                    131      99.95     94.00                                                      133.50                       79.94   94.10                                    141.50                       72.96   94.45                                    154      99.32     95.05                                                      178      97.00     96.28                                                      186      95.18     96.51                                                      ______________________________________                                    

Comparing the results obtained it is observed that, under the sameoperating conditions and with catalysts comprising the same atomicquantity of metals from groups VIII and IB, respectively, the catalystused in Example 1 (in accordance with the invention), comprisingpalladium and silver, is markedly more selective to begin with than thecatalyst based on platinum and copper used in Example 2 (not inaccordance with the invention). Moreover, the Pt--Cu catalyst requiredmore than 40 h of operation before the selectivity for propylene reached90%, whereas with the Pd--Ag catalyst a selectivity of 90% was reachedafter only 3.50 h.

A comparison of the degrees of conversion, moreover, shows that thePd--Ag catalyst used in Example 1 proves more stable over time than thePt--Cu catalyst of Example 2.

Example 3 (in accordance with the invention)

A catalyst comprising 0.5% by weight of Pd and 0.5% by weight of Agrelative to the weight of active charcoal employed was prepared usingthe same support and a procedure similar to that described in Example 1.

3.61 g (7.50 cm³) of this catalyst were introduced into a reaction tube(internal diameter=0.8 cm). The reactor containing the catalyst wassubsequently fed at a rate of 2.6 l/h (s.t.p.) with 1,2-dichloropropaneand with 10.3 l/h (s.t.p.) of hydrogen at 350° C. under 1.5 bar. Theresidence time was assessed as being 1.4 s.

After various intervals of time, the degree of conversion of1,2-dichloropropane and the selectivities for propylene and propane(defined as the molar fractions of 1,2-dichloropropane reacted which areconverted to propylene and to propane, respectively) were measured. Theresults of these measurements are reproduced in Table II.

Example 4 (not in accordance with the invention)

1,2-dichloropropane was converted under the same operating conditions asthose described in Example 3, but using 3.53 g (7.50 cm³) of a catalystcomprising 1% by weight of Pt and 0.5% by weight of Ag relative to theweight of active charcoal employed. This catalyst was applied to thesame support and under the same operating conditions as in Example 1,using an aqueous solution of H₂ PtCl₆.6H₂ O.

After various intervals of time, the degree of conversion of1,2-dichloropropane and the selectivities for propylene and propane weremeasured. The results of these measurements are likewise reproduced inTable II.

Example 5 (not in accordance with the invention)

1,2-dichloropropane was converted under the same operating conditions asthose described in Example 3, but using a catalyst comprising 1% byweight of Pt and 0.3% by weight of Cu relative to the weight of activecharcoal employed. This catalyst was applied to the same support andunder the same operating conditions as in Example 1, using aqueoussolutions of H₂ PtCl₆.6H₂ O and CuCl₂.2H₂ O.

After various intervals of time, the degree of conversion of1,2-dichloropropane and the selectivities for propylene and propane weremeasured. The results of these measurements are likewise reproduced inTable II.

                  TABLE II                                                        ______________________________________                                        EXAMPLE 3                                                                     (0.5% Pd--0.5% BXAMPLE 4    EXAMPLE 5                                         Ag)            (1% Pt--0.5% Ag)                                                                           (1% Pt--0.3% Cu)                                  DURA-        selectivity    selectivity  selectivity                          TION  conv.  mol %     conv.                                                                              mol %   conv.                                                                              mol %                                (h)   %      C.sub.3 H.sub.6                                                                      C.sub.3 H.sub.8                                                                    %    C.sub.3 H.sub.6                                                                    C.sub.3 H.sub.8                                                                    %    C.sub.3 H.sub.6                                                                    C.sub.3 H.sub.8             ______________________________________                                        0.50  92.9   93.0   2.5                                                       0.73                                    98.9 32.3 66.7                        1.55  96.2   94.4   1.7                                                       1.63                                    100  39.0 60.0                        2.97                                    99.4 48.2 50.6                        9.05                                    97.1 73.0 25.6                        17.96                    100  59.3 39.2                                       20.1  96.9   97.7   0.0  100  61.4 37.1                                       21.05                                   97.6 87.8 10.6                        44.72                    100  73.9 24.4                                       46.05                                   97.1 93.1 5.3                         50.1  97.1   98.5   0.0                                                       70.05                                   97.8 94.6 3.9                         74.1  97.2   98.5   0.0                                                       ______________________________________                                    

Comparing the results obtained under the same operating conditions andwith catalysts comprising the same atomic quantity of metals from groupsVIII and IB respectively, the catalyst comprising palladium and silveris to begin with markedly more selective for propylene than thecatalysts based on platinum and silver or copper. Under the operatingconditions described above, using the Pt--Cu catalyst, more than 70 h ofoperation are required before the selectivity for propylene reaches 95%.At this time, the quantity of propane produced is still of the order of4%. Employing the Pd--Ag catalyst, a selectivity for propylene of theorder of 95% is reached after barely 1.55 h. The quantity of propaneproduced is very low right from the beginning of the reaction andbecomes negligible after less than 20 h of operation.

Example 6 (in accordance with the invention)

A catalyst comprising 0.5% by weight of Pd and 0.5% by weight of Agrelative to the weight of the support employed was prepared by using aprocedure analogous to that described in Example 1, but using titaniumdioxide as support (grade HARSHAW No. Ti-0720T 1/8").

2.16 g (2.5 cm³) of this catalyst were introduced into a reaction tube(internal diameter=1.0 cm). The reactor containing the catalyst wassubsequently fed at a rate of 0.4 l/h (s.t.p.) with 1,2-dichloropropane,with 0.8 l/h (s.t.p.) of hydrogen and with 2.7 l/h (s.t.p.) of helium at350° C. under 1.5 bar. The residence time was assessed as being 1.5 s.

The degree of conversion of 1,2-dichloropropane was 100%, theselectivity for propylene 82% and the selectivity for chloropropenes(sum of the fractions of 1-, 2- and 3-chloropropenes) 18%.

Example 7 (in accordance with the invention)

For the conversion of 1,2,3-trichloropropane, a catalyst was employedcomprising 1.5% by weight of Pd and 3% by weight of Ag relative to theweight of active charcoal, obtained as described in Example 1.

1.31 g (2.5 cm³) of this catalyst were introduced into a reaction tube(internal diameter=1.0 cm). The reactor containing the catalyst was thenfed at a rate of 0.78 l/h (s.t.p.) with 1,2,3-trichloropropane, with3.12 l/h (s.t.p.) of hydrogen and with 3.9 l/h (s.t.p.) of helium at300° C. under 3 bar. The residence time was assessed as being 1.7 s.

The degree of conversion of 1,2,3-trichloropropane was 93% and theselectivity for propylene 99%.

Example 8 (in accordance with the invention)

In this example, a Pd--Sn catalyst was employed on a support of activecharcoal.

a) Preparation of the catalyst on the support

50.0 g of active charcoal (grade NC 35 sold by CECA) having a porevolume of 0.5 ml/g were introduced into a round-bottomed flask with 18.0ml of water and 17.0 ml of a solution containing 0.0147 g of Pd/ml(solution of PdCl₂ in 6M HCl). After a period of 60 minutes at roomtemperature, the impregnated active charcoal was dried under vacuum,initially at 80° C. and then at 100° C. After cooling to roomtemperature, 16.08 ml of a solution containing 0.0171 g of Sn/ml(aqueous solution of SnCl₄.5H₂ O) were introduced into the flask. Aftera period 60 minutes at room temperature, the impregnated active charcoalwas dried under vacuum, first at 80° C. and then at 100° C. Theimpregnated and dried active charcoal was then treated for 4 h at 350°C. with hydrogen. The catalyst thus obtained comprised 0.5% by weight(4.7 mmol) of Pd and 0.55% by weight (4.6 mmol) of Sn relative to theweight of active charcoal employed.

b) Conversion of 1,2-dichloropropane

4.5 g (10 cm³) of the catalyst described above were introduced into areaction tube (internal diameter=0.8 cm). The reactor containing thecatalyst was then fed at a rate of 3.0 l/h (s.t.p.) with1,2-dichloropropane, with 21.0 l/h (s.t.p.) of helium and with 6 l/h(s.t.p.) of hydrogen at 300° C. under 3 bar for several hours. Theresidence time was assessed as being 1.7 s.

After 8.5 hours and 14.5 hours of operation, a sample of the productsleaving the reactor was taken and was analysed by gas chromatography,and the degree of conversion of 1,2-dichloropropane and the selectivityfor propylene were measured. The results of the measurements arereproduced in Table III. It can be seen in this table that the degree ofconversion is greater than or equal to 95% and that the selectivity forpropylene is 96% and that, after 14.5 hours of operation, the catalystis still as active and selective as after 8.5 hours of operation.

Examples 9 and 10 (not in accordance with the invention)

1,2-dichloropropane was converted under the same operating conditions asthose described in Example 8, but employing catalysts comprising 1% byweight of Pt and 0.3% by weight of Cu (Example 9) and 0.5% by weight ofPt and 0.3% by weight of Sn (Example 10), the contents being expressedrelative to the weight of active charcoal. These catalysts were appliedto the same support and under the same operating conditions as inExample 8, using aqueous solutions of H₂ PtCl₆.6H₂ O, CuCl₂.2H₂ O andSnCl₄.5H₂ O.

After intervals of time of 8.5 hours and 14.5 hours, the degree ofconversion of 1,2-dichloropropane and the selectivity for propylene weremeasured as in Example 8. The results of these measurements are likewisereproduced in Table III.

Examples 11, 12 and 13 (in accordance with the invention)

1,2-dichloropropane was converted under the same operating conditions asthose described in Example 8, but employing a catalyst comprising 0.5%by weight of Pd and 0.97% by weight of Pb (Example 11), 0.5% by weightof Pd and 0.96% by weight of Tl (Example 12), and 0.5% by weight of Pdand 0.98% by weight of Bi (Example 13), the contents being expressedrelative to the weight of active charcoal. These catalysts were appliedto the same support and under the same operating conditions as inExample 8, using aqueous solutions of Pb(CH₃ CO₂)₂.3H₂ O, Tl(CH₃ CO₂)₃and Bi(NO₃)₃.5H₂ O.

After intervals of 8.5 hours and 14.5 hours, the degree of conversion of1,2-dichloropropane and the selectivity for propylene were measured asin Example 8. The results of these measurements are likewise reproducedin Table III.

                  TABLE III                                                       ______________________________________                                                   After operation for                                                                       After operation for                                               8.5 h       14.5 h                                                                  conversion                                                                             selectivity                                                                          conversion                                                                           selectivity                           EX.    CATALYST  %        mol %  %      mol %                                 ______________________________________                                        8      Pd-Sn/C   95       96     96     96                                    9      Pt-Cu/C   98       61     99     81                                    10     Pt-Sn/C   99       58     98     77                                    11     Pd-Pb/C   96       94     96     97                                    12     Pd-Tl/C   90       98     97     97                                    13     Pd-Bi/C   91       96     86     97                                    ______________________________________                                    

Comparing the results obtained, it is observed that, under the sameoperating conditions, the catalysts used in Examples 8, 11, 12 and 13(in accordance with the invention), comprising palladium, make itpossible to obtain a very good conversion of 1,2-dichloropropane, whileat the same time being markedly more selective with respect to propylenethan the platinum-based catalysts used in Examples 9 and 10 (not inaccordance with the invention).

Example 14 (in accordance with the invention)

In this example, Pd--Sn catalyst was employed on an alumina support.

The catalyst was prepared under the same operating conditions as inExample 8, employing an alumina support (alpha-alumina having a porevolume of 0.43 ml/g and a specific surface area of 3 m² /g).

1,2-dichloropropane is converted under the same operating conditions asthose described in Example 8, but using 9.1 g (10 cm³) of a catalystcomprising 0.5% by weight of Pd and 0.55% by weight of Sn relative tothe weight of alumina.

After intervals of 8.5 hours and 14.5 hours, the degree of conversion of1,2-dichloropropane and the selectivity for propylene were measured asin Example 8. The results of these measurements are reproduced in TableIV.

Example 15 (not in accordance with the invention)

1,2-dichloropropane was converted under the same operating conditions asthose described in Example 14, but employing a catalyst comprising 0.5%by weight of Pt and 0.3% by weight of Sn relative to the weight ofalumina. This catalyst was applied to the same support and under thesame operating conditions as in Example 14, using an aqueous solution ofH₂ PtCl₆.6H₂ O.

After the same intervals of time, the degree of conversion of1,2-dichloropropane and the selectivity for propylene were measured asin Example 14. The results of these measurements are likewise reproducedin Table IV.

                  TABLE IV                                                        ______________________________________                                                   After operation for                                                                       After operation for                                               8.5 h       14.5 h                                                                  conversion                                                                             selectivity                                                                          conversion                                                                           selectivity                           EX.    CATALYST  %        mol %  %      mol %                                 ______________________________________                                        14     Pd-Sn/Al.sub.2 O.sub.3                                                                  68       97     61     97                                    15     Pt-Sn/Al.sub.2 O.sub.3                                                                  66       88     54     78                                    ______________________________________                                    

Comparing the results obtained, it is observed that, under the sameoperating conditions, the catalyst used in Example 14 (in accordancewith the invention), comprising palladium, is markedly more selectivewith respect to propylene than the platinum-based catalyst used inExample 15 (not in accordance with the invention).

What is claimed is:
 1. A process for converting a chlorinated alkane ofthe formula C_(n) H_(2n+2-x) Clx in which n is an integer from 2 to 6and x is an integer from 1 to (2n+2) into at least one less chlorinatedalkene by reacting the chlorinated alkane with hydrogen in the presenceof a catalyst on a support, said catalyst comprising palladium and ametal M selected from the group consisting of silver, gallium, indium,thallium, germanium, tin, lead, arsenic, antimony, bismuth and mixturesthereof.
 2. The process according to claim 1, wherein the metal M isselected from silver, tin, lead, thallium and bismuth.
 3. The processaccording to claim 2, wherein the metal M is tin or silver.
 4. Theprocess according to claim 1, wherein the chlorinated alkane is chosenfrom chloropropanes.
 5. The process according to claim 4, wherein thechloropropanes are by-products formed in the production of allylchloride and/or epichlorohydrin.
 6. The process according to claim 4,wherein the chlorinated alkane is 1,2-dichloropropane.
 7. The processaccording to claim 1, wherein the support is active charcoal.
 8. Theprocess according to claim 1, wherein the quantity of palladium on thesupport is from 0.05% to 10% by weight relative to the weight of thesupport.
 9. The process according to claim 1, wherein the quantity ofmetal M on the support is from 0.05% to 10% by weight relative to theweight of the support.
 10. The process according to claim 1, wherein theratio by weight of palladium to the metal M is from 0.05 to
 20. 11. Theprocess according to claim 1, wherein the reaction is carried out at atemperature of from 150° to 450° C. under a pressure of from 1 to 30bar.
 12. The process according to claim 1, wherein the molar ratio ofhydrogen to chlorinated alkane is from 0.1 to
 40. 13. The processaccording to claim 1, wherein the reaction is carried out in the gaseousphase with a mean contact time between the gases employed and thecatalyst of from 0.5 to 30 seconds.
 14. The process according to claim1, wherein said support is active charcoal, said palladium on saidactive charcoal support is from 0.05% to 10% by weight relative to theweight of the support, said metal M is tin or silver, said metal M onsaid active charcoal support is from 0.05% to 10% by weight relative tothe weight of the support, the ratio by weight of palladium to the metalM is from 0.05 to 20, and said chlorinated alkane is1,2-dichloropropane.
 15. The process according to claim 1, wherein thereaction is carried out in the gaseous phase at a temperature of from150° to 450° C. under a pressure of from 1 to 30 bar, with a meancontact time between the gases employed and the catalyst of from 0.5 to30 seconds, and a molar ratio of hydrogen to chlorinated alkane of from0.1 to
 40. 16. A process for converting a chlorinated alkane into atleast one less chlorinated alkene by reacting the chlorinated alkanewith hydrogen in the presence of a catalyst on a support, said catalystconsisting of palladium and a metal M selected from the group consistingof silver, gallium, indium, thallium, germanium, tin, lead, arsenic,antimony, bismuth and mixtures thereof.
 17. The process according toclaim 16, wherein said support is active charcoal, said palladium onsaid active charcoal support is from 0.05% to 10% by weight relative tothe weight of the support, said metal M is tin or silver, said metal Mon said active charcoal support is from 0.05% to 10% by weight relativeto the weight of the support, the ratio by weight of palladium to themetal M is from 0.05 to 20, and said chlorinated alkane is1,2-dichloropropane.
 18. The process according to claim 16, wherein thereaction is carried out in the gaseous phase at a temperature of from150° to 450° C. under a pressure of from 1 to 30 bar, with a meancontact time between the gases employed and the catalyst of from 0.5 to30 seconds, and a molar ratio of hydrogen to chlorinated alkane of from0.1 to 40.